1. Field of the Invention
The present invention relates to an active infrared detection apparatus for use in security systems or the like, and more specifically to an active infrared detection apparatus that can maintain its reliability of operation when installed outdoors, even when the surrounding environment deteriorates.
2. Related Art
Conventionally, active infrared detection apparatuses for detecting the intrusion of an intruder into an alert territory, which are used in security systems, are known.
This type of active infrared detection apparatus generally includes a light-projecting apparatus including a light-projector, and a light-receiving apparatus including a light-receiver. The light-projecting apparatus and the light-receiving apparatus are installed at both ends of a straight-line alert territory, and an infrared light beam is emitted from the light-projector of the light-projecting apparatus toward the light-receiver of the light-receiving apparatus. Then, when the infrared light beam is interrupted by an intruder or the like and the amount of light received by the light-receiver changes, the apparatus activates for example a security camera or triggers an alarm call to a security company.
Incidentally, due to the influence of modulated light of fluorescent lamps or high frequency waves by AC radios or the like, it may happen that the active infrared detection apparatus does not operate properly. One example of this is the case that the light-receiving apparatus receives modulated light (noise signal) from a fluorescent lamp with about twice the frequency of the pulsed light of the infrared light beam from the light-projecting apparatus. In such an event, as long as the light of the fluorescent lamp is not blocked, the light-receiving apparatus continues to receive modulated light from the fluorescent lamp, even if someone intrudes into the alert territory and blocks the infrared light beam.
In this situation, it becomes impossible to distinguish between a state in which the pulsed light of the infrared light beam which is originally supposed to be detected is received and a state in which the noise signal is received. As a result, it becomes impossible for the active infrared detection apparatus to recognize that someone has intruded into the alert territory and it lapses into a state of “missed alarm”. Such a state of “missed alarm” is not limited to the case that the noise signal has the double frequency of the pulsed light of the infrared light beam, but can occur when the frequency of the noise signal is about an integer multiple or when it is a frequency higher than the pulsed light of the infrared light beam.
In view of this, an active infrared sensor, that, when installed in an environment with a noise signal, is capable of recognizing a deterioration of the environment and the risk of an missed alarm, has been proposed (e.g., JP 2001-235367A).
Meanwhile, when such an active infrared detection apparatus is installed outdoors, it happens that due to the seasonal or climatic conditions, frost or fog adheres to the infrared light beam transmission window portion of the cover. When that happens, through the attenuation of the infrared light beam, the amount of light received by the light-receiving elements decreases notably and there is a risk of a “false alarm”, in which it is determined that there is an intruder, even when no intruder is present.
In view of these problems, an infrared light beam sensor has been proposed that can prevent the adhering of frost or fog by providing a hood on top of the infrared light transmission window portion of the cover, and furthermore providing the infrared light window transmission portion with a planar heater (see, for example, JP H8-171679A).
However, the conventional art, such as the above-mentioned JP H8-171679A, is not effective when there are factors, such as fog or rain that attenuate the infrared light beam at a location away from the active infrared detection apparatus.
Furthermore, in view of such factors that attenuate the infrared light beam, in order to let an infrared light beam of sufficient intensity reach the light-sensitive elements even under such circumstances, it is conceivable to set the infrared light beam to strong intensity in advance. However, when no factor that attenuates the infrared light beam occurs, there is the possibility that due to the strong intensity of the infrared light beam a missed alarm occurs due to the reflection light from objects that are not to be detected in the vicinity of the active infrared sensor.